Water treatment plants generally process raw water, which includes particulate impurities, into potable water. In this process chemicals are introduced into the raw water for killing harmful bacteria and for an initial removal of particulate impurities suspended in the water. The chemicals and the water are mixed and the mixture is then passed via a flocculator to a sedimentation basin. In the sedimentation basin the water flows evenly and slowly and remains for a sufficient time so that particulate impurities entrapped in coagulated flocs can settle to the floor of the basin for subsequent removal. From the sedimentation basin the water passes to the water filters and hence to a suitable potable water storage for introduction in the water distribution system.
A major part of impurities in the raw water is removed in the sedimentation basin. This removal is performed by introducing a chemical, such as aluminum sulphate which coagulates into particles around particulate impurities. The particles are of such small size that they remain suspended in the water. To effectively settle them they must be combined into larger flocs which is accomplished by bringing as many particles as possible into contact.
It is conventional to provide flocculators which agitate the water as it enters the sedimentation basin. This agitation causes turbulence and an intimate mixing and brings particles into mutual contact so that flocs are formed. It is known that increased agitation enhances the formation of larger flocs which in turn shortens the required time to settle them in the basin. A shorter sedimentation time, in turn, makes it possible to shorten the length of the settlement basin and thus lowers the costs for the water treatment plant.
It is also known, however, that the increase in agitation is limited since the higher relative water speeds have a tendency to tear apart or separate the flocs into smaller agglomeration of particles. Thus, there is an optimal point in the operation of prior art flocculators which limits the maximum floc size that is attainable. Repeated attempts have been made to decrease the sedimentation times by increasing the floc density. However, an increase in the floc size, if it can be attained, would be more desirable since the sedimentation rate increases at a faster rate with an increase in the particle size.
A further and very costly drawback of prior art flocculators is their relatively large power consumption. The flocculators normally comprise rotatable hubs from which radial arms extend. Paddles made from wire mesh, flat boards or the like are mounted to the ends of the arms. To obtain the required water agitation the flocculators are power driven. Their relatively large size and their substantial water resistance consumes large amounts of energy. To reduce the energy consumption the effective flocculator areas opposing the water flow and causing the water agitation, such as the wires in the wire mesh or the boards, represent a relatively small proportion of the water flow cross section. Although in the past this was not thought to be significant, such a construction of the flocculators in fact reduces the flocculation effect as will be pointed out more fully hereinafter.